After finishing a race I can cycle through all cars, both cars that are still racing and cars that have finished, but when there is only one more car still racing I can only cycle through cars that have finished the race.

I can't reproduce this. I completed a race offline (finished 3rd), waited till 7 out of 8 players finished, then by pressing down arrow thrice, I reached the 2nd, 1st, then the 8th player that was still racing. Up arrow works too.

Note that the cars are cycled through according to their position, so it's possible you had to cycle through all the finished cars before you reached the last one.